1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a displacement between two objects and a method and an apparatus for measuring a gap distance between two objects. More particularly, the invention relates to a method and an apparatus for measuring a displacement between a mask and a wafer for relative alignment between them, in an exposure apparatus for the manufacture of semiconductors, and also to a method and an apparatus for measuring a gap distance between a mask and a wafer for relative alignment between them, in an exposure apparatus.
2. Description of the Related Art
In the process of manufacturing a semiconductor device such as a VLSI, an exposure apparatus transfers the circuit pattern of the device onto a wafer. In an X-ray exposure apparatus, in particular, X-rays are applied to the wafer through a mask having the circuit pattern, thereby transferring the image of the pattern onto the wafer. In order to transfer the circuit pattern onto the wafer, it is necessary to align the mask and the wafer, which face each other, in their facing direction, and to provide a desired gap between them.
A method and an apparatus for executing the alignment, gap setting, and relative alignment with relatively high accuracy are disclosed in, for example, Published Unexamined Japanese Patent Application No. 62-261003. In this proposed method or apparatus, relative alignment is effected by the optical heterodyne interference method using one-dimensional diffraction gratings. According to this method, the mask is provided with a one-dimensional diffraction grating and a window, and the wafer with a reflecting surface and another one-dimensional diffraction grating.
First, the alignment is executed. In doing this, two laser light beams individually having frequencies f1 and f2 are applied in the .+-.1-order direction to the one-dimensional diffraction grating of the mask. These light beams are transmitted through the one-dimensional diffraction grating of the mask to be diffracted thereby, reflected by the reflecting surface of the wafer, and transmitted again through the one-dimensional diffraction grating of the mask to be diffracted thereby. Thereupon, the light beams are changed into diffracted interference light beams I.sub.M, thus appearing distributed one-dimensionally. The light beams transmitted through the window of the mask, on the other hand, are transmitted through the one-dimensional diffraction grating of the wafer to be diffracted thereby, and transmitted again through the window of the mask, thus appearing as diffracted interference light beams I.sub.W distributed one-dimensionally. The phase difference .DELTA..phi..sub.X between light beams of I.sub.M (0, 0) and I.sub.W (0, 0) orders, out of the diffracted interference light beams I.sub.M and I.sub.W, is detected. Since the phase difference .DELTA..phi..sub.X corresponds to a displacement between the mask and the wafer, the displacement can be determined by calculation. The mask and the wafer are aligned on the basis of this determined displacement.
Subsequently, the gap setting between the mask and the wafer is executed. In doing this, the light beam with the frequency f1 is incident in the +1-order direction, as in the case of the alignment, while the light beam with the frequency f2 is incident in the +3-order direction. Thereupon, a light beam of I.sub.W (-2, 0) order is detected which is diffracted and caused to interfere with each other on the same optical path for the alignment. The phase difference .DELTA..phi..sub.Z between the light beams of I.sub.W (-2, 0) and I.sub.M (0, 0) orders is detected. Since the phase difference .DELTA..phi..sub.Z corresponds to the gap distance between the mask and the wafer, the gap distance can be determined by calculation. The predetermined gap is set between mask and the wafer on the basis of this determined gap distance. More specifically, a gap z may be expressed as follows: EQU z=Zp.sup.2 /.pi..lambda.,
where Z=1/8.multidot.(.DELTA..phi..sub.Z +2X), X=2.pi..DELTA.x/p, p is the pitch of the diffraction grating, and .lambda. is the wavelength of the light.
Thus, the displacement and the gap can be measured in accordance with the phase differences, and the alignment and the gap setting can be executed on the basis of the measured values.
According to this method, the diffraction grating formed on each of the mask and the wafer is a one-dimensional diffraction grating which has a plurality of parallel stripes extending at right angles to the aligning direction, so that the diffracted light beams are one-dimensionally distributed in the aligning direction. Further, the respective diffraction gratings of the mask and the wafer are deviated from each other in the direction perpendicular to the aligning direction, so that the diffracted light beam of I.sub.W (-2, 0) order appears in close vicinity to the diffracted light beam of I.sub.M (-2, 0) order which is generated simultaneously therewith. More specifically, these two diffracted light beams are emitted at a very short distance of about 100 .mu.m from each other, and partially overlap and interfere with each other. Accordingly, the diffracted light beam of I.sub.W (-2, 0) order cannot be detected selectively and separately from the diffracted light beam of I.sub.M (-2, 0) order. If the light beam of I.sub.W (-2, 0) order is detected interfering with the light beam of I.sub. M (-2, 0) order, therefore, the gap between the mask and the wafer cannot be set with high accuracy.